Single frequency laser

ABSTRACT

This invention relates to generally to semiconductor devices, for example lasers and more particularly to single frequency lasers and is directed at overcoming problems associated with the manufacture of these devices. In particular, a laser device is provided formed on a substrate having a plurality of layers ( 1,2,3,4,5 ), the laser device comprising at least one waveguide (for example a ridge) established by the selective removal of sections of at least one of the layers. The ridge ( 100;101 ) has at least one defect defining region ( 104 ), the at least one defect defining region of the ridge defining a defect in the ridge. The width of the ridge is greater in the at least one defect defining region of the ridge than in adjacent sections of the ridge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior application Ser. No.10/399,070, filed Apr. 10, 2003.

FIELD OF THE INVENTION

The invention relates generally to semiconductor devices, in particularwaveguides, for example lasers and more particularly to single frequencylasers and a method, production layout or mask for the manufacture ofsame.

BACKGROUND OF THE INVENTION

Semiconductor waveguides, for example lasers or laser diodes are wellknown in the art with many applications in data communications,telecommunications, metrology and sensing. The most common method inachieving single frequency operation in a semiconductor laser is byetching a grating layer into the semiconductor and burying that gratinglayer using epitaxial overgrowth. As the grating pitch is of the orderof the wavelength/refractive index, either holographic or electron beamtechniques are required for their definition. In addition in many casesthe added complexity of material overgrowth makes those commontechniques expensive.

One of the simpler and more reliable laser devices available is the selfaligned ridge laser. Such a device is described in U.S. Pat. No.5,059,552.

FIG. 1 illustrates a typical example of a “ridge” laser or lasing device7. The representation of a finished ridge laser device is a simplifiedversion for the purposes of explanation. In operation, light 9 isprimarily emitted from shaded region 8. The formation of a “ridge” 6 iseffected by etching into a layered material, as shown in FIG. 2,comprising a series of at least four epitaxial layers 1, 2, 3, 4 formedon a semiconductor substrate 5. For a n-type substrate 5, the top layer1 which is the contact layer comprises p-type material, the second layer2 is a cladding layer and also comprises p-type material. The thirdlayer 3 typically comprises a number of undoped active layers which areused for light guiding and gain purposes and may be composed of bulk,quantum well or quantum dots. The fourth layer 4 is an n-type claddinglayer. It will also be understood that suitable etch stopping layers mayalso be incorporated into the structure 20. As with other semiconductordevices, typically a large number of ridge lasers are formed on a singlesemiconductor wafer and subsequently divided. Accordingly, thestructures illustrated should be taken as only a part of a largersemiconductor body.

For the case of a InP laser emitting in the 1.2-2.0 μm wavelength range,the epitaxial top layer 1 is typically InGaAs, with the second andfourth layers and the substrate typically InP, with layer 3 typicallycontaining InGaAsP and/or InGaAlAs. Other material combinations are alsopossible. It will also be appreciated that alternative semiconductormaterials, e.g. those based on GaAs, GaSb, or GaN would incorporatedifferent epitaxial layers and could require alternative etchants.

The process of manufacturing a “ridge” laser, as illustrated insimplified form in FIG. 3, commences with the formation (30) of theoutline of the ridge in a layer of resist material 40, as shown in FIG.4, on the top layer 1, using a suitable lithographic technique. Theshape of the resist material is determined by the mask used in thelithographic process. The subsequent outline of the ridge formed will bedetermined by the outline of the mask.

The next step in the process is to etch 31 the structure to remove thetop layer and part of the second layer in regions not covered by theresist material. The thickness (t) of the remaining portion of layer 2in the region which has been etched 51, 52 contributes to thecharacteristics of the finished “ridge” laser. In regions covered by theresist material, the top layer and second layer are substantiallyunaffected by the etching process, thus leaving a raised surface or“ridge” effectively matching the mask outline. The width of the ridge(w) formed matches that of the outline formed by the resist material 40.

Single spatial mode output is essential for many applications and istypically obtained by an appropriate choice of ridge width (w) and etchdepth, or more correctly the remaining thickness (t). Typically,imposing a ridge width (w) in the range of 2-4 μm while t is of theorder of 0.1-0.5 μm. Achieving this single mode is an objective of the‘ridge’ and is relatively easily obtainable using conventionaltechniques.

The third step 32 in the process is to apply a dielectric coating 60over the structure, as shown in FIG. 6 a. The next step 33 involvesremoving a portion 61 of the dielectric material 60 covering the topsurface of the ridge, as illustrated in FIG. 6 b, using a conventionaletching technique. The final step 34 is to apply a metal contact layer62 on the portion of the ridge 6 not covered by the dielectric materialto form a metal contact 62, as shown in FIG. 6 c. Other steps not showninclude an alloy and thinning of the substrate to approximately 100-120um, a further metal coating step to apply a metal contact layer to thesubstrate with subsequent alloy, a cleaving step in which the ridgelaser is cleaved at a particular point to define its end, and thebreaking up of the wafer into individual ridge laser devices.

Although such “ridge” laser devices are reliable, there is a tendency ofsuch structures to operate with multi-longitudinal modes. The work ofDiChiaro (L. DeChiaro, J. Lightwave Technology, Vol 8 November 1990 pp1659-1669, J. Lightwave Technology, Vol 9 August 1991 pp 975-986Z)showed that the introduction of a defect at a fractional position of thecavity length could convert multi-longitudinal modes into a singlelongitudinal mode. This method is however rather crude and introducesdamage into the laser. Further to this, Patterson et al and Kozlowski etal (B. D. Patterson et al, Microelectronic Engineering, Vol 27 1995, pp347-350, D. A. Kozlowski et al, Electon. Letters, Vol 31, April 1995, pp648-650) used focused ion beam etching to create a series of holes alongthe length of a laser cavity. Kozlowski monitored the spectrum of thelaser during this process, and showed that the etching allowed for animproved spectral performance (Side Mode Suppression Ratio—SMSR) throughthe enhancement of the effect by several ‘defects’ acting together.However, this work was on fully processed lasers, i.e. post laserfabrication, and has limited commercial application. Further work by thepresent inventor (B. Corbett and D. McDonald, Electron Letters, Vol 31,December 1995, pp 2181-2182) showed that the integration of the defectcould be incorporated into the standard process sequence for ridgewaveguide lasers.

The integration of a suitable defect may be incorporated using electronbeam lithography by the placement of a gap in the mask forming theridge, thus resulting in an omission of a section of resist material.Accordingly, the etching process forms a slot 70 between two sections71, 72 of the ridge as shown in FIG. 7. More than one slot may berequired to achieve a desired characteristic in each ridge laser device.An implementation, as published by the present inventor [supra], havinga slot width “s” of 0.5-1.0 μm allows for accurate reproduction usingelectron beam lithography. The width of the slot “s” is determined bythe gap width in the mask forming the ridge. The etch depth (d) of theslot is the same for both the spatial mode and the longitudinal mode,resulting from the processing of both ridge and slot in the same processstep, i.e. the depth (d) of the slot 70 equals the height (h) of theridge. It will be noted that the slot formed is dimensionally along thelongitudinal axis of the ridge.

Electron beam lithography is a ultra-high definition direct-writelithographic process where is accomplished by omitting to write (or insome processes writing) a pattern of electrons onto a resist layer.

Conventionally, the length of the ridge and accordingly the length ofthe laser are defined by marking the edge of the processed material witha diamond scribe perpendicular to the ridge. Increasing sophisticationof tools now permits an absolute marking accuracy of a few microns withrelative marking accuracy less than 1 μm. However, in order to haveaccurate absolute markings with respect to the slot location, a specialcleave feature is defined lithographically. This is achieved using anon-selective dry etch through the active layers and a specialcrystallographic wet etch that ends in a sharp line; this being theintersection between two crystal planes, as shown in FIG. 7. The edgethen serves as the location of the cleave plane and hence the end of thelaser (referred to as the facet). For example, in FIG. 7 the ridge laserdevice would have its end defined by the line a-a after the cleavingprocess. The appropriate wet etchant for InP substrate is HCl. Again, itwill be appreciated that the etchants described hereinbefore arespecific to an InP substrate and that alternative substrates such asGaAs may require different etchant materials. The registration of theslot with respect to the cavity length is obtained by the highresolution of the direct write electron beam lithography system. Thecleave feature is formed in a separate process to the formation of theridge and slots.

As detailed above, this existing technique is implemented using directwrite electron beam lithography, which is highly accurate in itsdefinition of features and in referencing between lithographic levels.However, use of this technique is slow, expensive and does not deliversufficient cost benefit to be favoured over alternative techniques forsingle frequency lasers such as Distributed Feedback (DFB) devices. Dueto the large costs associated with purchasing and running directelectron beam lithographic equipment, it will be appreciated that it isnot commercially feasible to use such equipment for production purposes.To be commercially viable the process needs to be implemented with morecost efficient techniques such as optical lithography. In the presentapplication, the use of the word optical is intended to include anylithographical process using the projection of a resist modifying fluxthrough suitable masking apertures, and includes the use of visiblelist, deep UV, or scattering electron beam lithography. The use ofoptical lithography has, however, associated shortcomings such asresolution and alignment accuracy. In particular, to the resolutionrequirement to define, for example, a ridge of width of the order of 3μm, having a slot with a gap of the order of 0.5 μm, is not obtainableusing conventional optical lithographic techniques.

Using optical lithography to produce a ridge outline having a gap in theresist layer is ineffective. The resist pattern 82 becomes rounded inregions corresponding to the corner regions of the mask 80 and leads toan ill-defined resist surface pattern compared to the results 81achieved using direct write electron beam lithography, as shown in FIG.8. This rounding of corners in resist patterns as reproduced on a ridgesubsequently by etching, results in a general degradation in the singlefrequency performance of the resulting “ridge” laser. If poor contact ismade between the mask and the imaging resist during pattern transfer theresultant degradation in corners can be more severe due to diffraction,which results in further degradation in the single frequency performanceof the laser device.

It is important for InP lasers that the ridges are aligned parallel tothe major flat (crystal planes) of the wafer. It will be understood thatall wafers have the crystal axes identified by the roundness beingflattened, which is what is meant by the term crystal flat.

Accordingly, it would be beneficial if a “ridge” laser could be designedhaving single frequency performance similar to that available usingdirect write electron beam lithography but which could be manufacturedusing conventional optical lithography techniques.

A further important alignment is in the referencing between atopographic feature for example a slot in a waveguide, and the laserfacet. The laser facet is the break in the laser which is along acrystal axis and which is the mirror providing feedback into the laser.The position of the laser facet is defined by the cleave feature asshown in FIG. 7. The reflectivity of the facet may be changed by theapplication of coatings. It is advantageous to have this referencing asaccurate as possible for best reproducibility in device performance. Thecleave feature is a notch formed by etching. FIG. 7 illustrates that theridge is the only structure extending above the etched surface and thatthe cleave feature starts at the etched level. It will be appreciatedthat conventionally, whilst areas adjacent to the ridge may be etched,other regions of the semiconductor structure may be of similar height tothe ridge. In these situations the cleave feature will extend from thetop layer through to the substrate. In the direct write electron beamprocess prior art the cleave feature is formed in a different step ofthe process to the ridge and slot. Alignment of the slot and cleavefeature is achieved by the resolution available through electron beamlithography.

It would further be beneficial if conventional optical lithographymanufacturing processes could be modified to allow in manufactureregistration or alignment of slots or other topographic features to thecleaving feature and hence facet of a opto-electronic device, forexample a laser.

Accordingly, there is a need for an improved semiconductor device, e.g.ridge laser and method of making same.

SUMMARY OF THE INVENTION

This need and others are satisfied by the present invention, in which afirst embodiment of the invention provides a semiconductor device forexample an opto-electronic laser device formed on a substrate having aplurality of layers with a topographical feature, for example a ridgeestablished by the removal of sections of at least one of the layers,the ridge comprising at least one elongate section having at least oneassociated defect defining region, the at least one defect definingregion defining a defect in the topographical feature, wherein the widthof the topographical feature is greater in the at least one defectdefining region than in adjacent regions of the at least one elongatesection.

It will be understood by those skilled in the art that in the context ofthe present invention a defect is an element which effects a change inthe reflective index or gain of a structure, feature or waveguide.

Each defect defining region may comprise at least one side portionextending perpendicular to the longitudinal axis of the elongate bodyportion. Preferably, each defect defining region has two side portionsextending in opposite directions from the elongate body portion.

The topographical feature may comprise at least one pair of opposingT-shaped ridge sections disposed along a longitudinal axis.

In one embodiment, the topographical feature is a ridge. In a preferredvariation, the defect comprises an aperture formed in the ridge. In thispreferred embodiment, the height of the ridge may be substantially thesame as the depth of the aperture defined in the defect defining region.This provides the advantage of manufacturing the ridge and aperture inthe same step. Alternatively, the depth of the aperture defined in thedefect defining region is different to the height of the ridge and ispreferably less than the height of the ridge. This provides theadvantage of allowing independent control of wavelength (slots) andspatial mode (ridge).

In an alternative variation, the defect is formed by Ion Implantation.In a further alternative variation, the defect is formed using anImpurity Induced Layer disorder. Using the extra width in the associateddefect defining region prevents damage to the junction region of laserdevices in the vicinity of the defect.

In a second embodiment, a semiconductor device, for example a laser, isprovided, the device comprising at least one ridge formed on asubstrate, the at least one ridge laser comprising at least one pair ofopposing T-shaped ridge sections disposed along a longitudinal axis.Each opposing pair of T-shaped ridge sections are preferablysubstantially separated by an associated defect region, for example asuitably formed aperture in the ridge. The device may comprise aplurality of ridges formed on the same substrate. Optionally, each ofthe ridges, for example which may be ridge lasers, may be designed bycareful selection of defect positioning and/or size, to providedifferent characteristics, e.g. wavelength, for each of the ridges ofdevices. The invention also extends to a optical, for example lasersystem, comprising the devices described having a plurality of ridges,each ridge being optically coupled to an associated light guide, or anoptical multiplexer coupled to the associated light guides and adaptedto receive the light outputs from each of the ridges and to combine thelight outputs to provide a single light output. In a preferredvariation, each ridge forms an associated ridge laser.

According to a third embodiment of the invention, a method ofmanufacturing an optoelectronic device, for example a ridge laser havingone or more defect regions on a semiconductor body comprising asemiconductor substrate having a plurality of layers disposed thereon,the method comprising the steps of:

performing a first etching to substantially define a ridge on thesemiconductor body,

using a second etching to provide independent control of the defectdepth with respect to the ridge height.

In one variation of this third embodiment, the first etching forms anelongate ridge of substantially uniform height above the etched surfaceof the semiconductor body and the second etching forms an aperture inthe elongate ridge.

In an alternative variation, the first etching forms an elongate ridgeof substantially uniform height above the etched surface and having anaperture defined therein, and wherein the second etching increases theheight of the ridge relative to the aperture depth. The layers may beepitaxial layers.

In a further embodiment of the invention a method is provided formanufacturing a semiconductor device for example a laser, from asemiconductor body comprising a semiconductor substrate having aplurality of layers disposed thereon comprising the steps of:

forming a first resist layer on the surface of the semiconductor body,the first resist layer defining the upper surface of an elongate ridgeportion having a uniform width,

performing a first etching of the semiconductor body to form an elongateridge of uniform height above the etched surface of the semiconductorbody,

forming a second resist layer on the surface of the semiconductor body,the second resist layer defining the surface outline of a slot in theelongate ridge, and

performing a second etching of the semiconductor body to form anaperture in the elongate ridge. Preferably, the aperture is in the formof a slot transverse to the longitudinal axis of the ridge. The secondetching may be such that the depth of slot formed may differ from theheight of the elongate ridge. The first resist layer in defining theelongate ridge portion includes at least one side portion extendingperpendicular to the longitudinal axis of the ridge adjacent to the slotformed in the ridge. The step of forming a second resist layerpreferably utilises a mask that defines an etching region narrower thanthe width of the elongate portion of the ridge.

The invention also provides a method of manufacturing a semiconductorlaser from a semiconductor body having a semiconductor substrate havinga plurality of, for example epitaxial layers disposed thereon comprisingthe intermediate step of:

forming a resist layer on the surface of the semiconductor body, theresist layer defining a surface ridge portion having at least oneassociated defect surface portion, the defect surface portion having atleast one side surface portion extending perpendicular to thelongitudinal axis of the elongate surface ridge portion, performing anetching of the semiconductor body to form a ridge having an elongateportion and at least one defect portion, the defect portion having atleast one side portion extending perpendicular to the longitudinal axisof the elongate ridge portion. The defect portion may have an apertureformed therein.

The invention further provides a method of aligning between lithographiclevels of a semiconductor body comprising the steps of:

forming a topographical outline, for example of a waveguide in the formof a ridge, having at least one defect region defined therein and theoutline of a cleave marking region in a first layer of resist materialon the surface of the semiconductor body,

performing one or more processes to form the topographical feature and acleave marking notch from the topographical outline,

forming the outline of a second cleave marking region in a second layerof resist material on the surface of the semiconductor body, where thesecond cleave marking region substantially overlaps the notch formed inthe cleave marking region,

performing a second etching into the active layers of the semiconductor,

performing a third etching using a crystallographic etches as to form aself-aligned feature.

Further objects, features and advantages of the present invention willbecome apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail having regard to theaccompanying drawings in which:

FIG. 1 is a schematic of a conventional prior art ridge laser structure,

FIG. 2 is a schematic showing a section of semiconductor body fromwhich, an optoelectronic device, for example a ridge laser may beconstructed,

FIG. 3 is a flowchart illustrating the steps in the process ofmanufacturing a conventional ridge laser of the type shown in FIG. 1from the semiconductor body of FIG. 2,

FIG. 4 is a schematic showing the semiconductor body of FIG. 2 having anapplied layer of resist pattern,

FIG. 5 is a schematic showing the structure of FIG. 4 after an etchingprocess has been performed,

FIG. 6 (a-c) represents profile views of the structure of FIG. 4 anddemonstrate the final steps in the manufacturing process for a ridgelaser,

FIG. 7 shows a pictorial view of a further example of a prior art ridgelaser device,

FIG. 8 is a pictorial representation of masks and resulting resistpatterns,

FIG. 9 is a representation of a mask and resist feature according to anembodiment of the present invention,

FIG. 10 is a pictorial representation of a ridge laser according to thepresent invention,

FIG. 11 is a process according to the invention,

FIG. 12 illustrates some of the steps of the process of FIG. 11,

FIG. 13 is a further process according to the invention,

FIG. 14 is a pictorial representation of a semiconductor structure atdifferent stages of the process shown in FIG. 13, and

FIG. 15 is a further process according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 8 have been discussed with reference to the prior art in thesection “Background of the Invention”.

FIG. 9 a represents an exemplary lithographic mask 85 according to theinvention for the production of a topographical outline, in the presentexample a ridge surface outline 86, in a masking layer, for example ofresist material, on a semiconductor structure using conventionallithography. The lithographic mask 85 may be made in accordance withconventional techniques, such as using electron beams to define apattern in chrome on glass or quartz. It is advantageous to use thehighest possible resolution in the manufacture of the mask. The maskcomprises an opposing pair of ‘T’ shaped regions 87 a, 87 b, each ‘T’shaped surface region 87 a, 87 b having an elongate section 90 and adefect defining region 91. The defect defining region 91 of FIG. 9 a isidentifiable as the transverse bar section of the ‘T’ shape. A gap ifformed in the mask between the defect defining regions of the opposing‘T’ shaped regions. The width of the ridge outline (w) the presentexample is substantially constant along the elongate section 90 of the‘T’ shaped regions of the mask. The mask is wider in the defect definingregions 91 of the mask than adjacent regions of the mask. It will beappreciated that the length of the elongate section need not besubstantially constant. The defect defining region 91 is wider becauseof side portions extending perpendicular to the elongate sections. Inpractise, more than one pair of opposing ‘T’ shaped sections may beprovided in each ridge outline.

Differing from the prior art, the present invention provides asubstantially non-linear mask, such that at the position where a slot(defect) is required in the ridge the mask includes features that extendtransversely from the longitudinal axis of the ridge. As describedpreviously, corner regions have the most detrimental effect on patterntransfer. The corners of the mask of the present invention are offset toeither side of the longitudinal axis of the ridge mark (elongateportion) with the effect that the resultant corners of the slot (defect)formed in the ridge are moved away from the critical gap region, leadingto a more reliable and reproducible pattern transfer in the criticalguiding region.

The resulting resist layer 86, illustrated in FIG. 9 b, formed on asemiconductor body using the mask 85 and conventional opticallithography techniques demonstrates rounding effects. However therounding effects are located in the side portions 95 a, 95 b, 97 a, 97 bof the defect defining region and away from the elongate portion 94 ofthe ridge and the critical region 96 of the gap 98 formed between thetwo opposing ‘T’ shaped ridge patterns.

The resulting topographical feature, i.e. the ridge, formed by anetching process is shown in FIG. 10, in which the rounding effects fromthe resist pattern have been reproduced in the side portions of thedefect defining regions of the ridge. However, it is clear that theopposing surfaces of the T shaped sections 100, 101 of the ridge do notdemonstrate rounding in the critical region of the ridge adjacent to theslot 104 formed between the two opposing ‘T’ shapes. Thus the roundingeffect has been moved away from the critical region of the ridge andinto the side portions 102 a, 102 b, 103 a, 103 b. The width and depthof the side portions should be selected such that there is no modeconversion and that excessive current spreading does not occur. It willbe appreciated that the primary purpose of the side portions is toremove rounding effects from the critical central section of theelongate structure of the ridge. In the example shown, the sloteffectively separates the two opposing ‘T’ sections. It is however,sufficient that an aperture be provided to separate opposing criticalregions of the ‘T’ sections. While the example illustrated is for a Tshape, it will be appreciated by those skilled in the art that any otherconfiguration which moves the corners away from the critical regionwithout necessarily changing the essential width of the elongate sectionof the ridge will be equally applicable.

As would be expected, in the example shown, the ridge formed is ofuniform height and the depth of the slot equals the height of the ridge.

The use of a wider defect region than the elongate portions of thetopographical feature, e.g. a ridge laser, may also be beneficial inother processes where the defect is formed using a method other than theetching process described.

For example, if the defect is formed using a Selective Ion Implantationtechnique, in which the area where a defect is required is selectivelybombarded with hydrogen ions. This bombardment of hydrogen ions altersthe refractive index of the material in the defect region thusintroducing a defect. In this case, the extra width of the ridge in theregion of the defect prevents accidental damage to the underlying layersin the region of the defect. In the absence of this extra width, spillover of hydrogen ions from the defect area could enter underlying layersin the area adjacent to the defect region ridge and alter thecharacteristics of the underlying areas, which could affect theperformance of the laser.

For similar reasons, the use of defect defining region which is widerthan the elongate sections of the ridge may have advantages in otherprocessing techniques used to form the defects, e.g. Impurity InducedLayer Disorder Techniques (IILD), which use localised heating to createa defect.

A further aspect of the invention provides a process for the productionof semiconductor devices, for example lasers, and for the forming of acleave feature in the semiconductor body. This technique relies on theavailability of selective etchants for the different layers thatcomprise the epitaxial material. The example which follows relates toIndium Phosphide (InP) based lasers where typically an InGaAs(P) activelayer system is sandwiched between InP n- and p-doped layers on ann-type InP substrate. It will be appreciated that the layers and devicedescribed herein are illustrative only, and it is not intended to limitthe application of the present invention to such specifics. For example,the ‘active’ region may consist of 10's of individual layers, comprisedfor example of quantum wells. Semi-insulating substrates could also beused, as could “p type” substrates, but as is known within the art “ptype” substrates are not of as high a quality. Similarly, the techniquemay be used for other optoelectronic devices.

The process, shown in FIG. 11, commences with the formation 110 of acleave marking region 120 in area of resist material at the same time asmarking a topographical feature, i.e. the ridge pattern 121 a, 121 b inthe resist material as shown in FIG. 12 a (the resist material is theshaded area).

The next step 111 is to use an etching process to form the ridgesections 123 a, 123 b which are separated by a defect regions (slot) asshown in FIG. 12 b. The etching also forms a cleave marking notch 125.This first etching process may for example be a non-selective plasmaetch to etch into the InP upper cladding layer (layer 2). It will beappreciated that any etch which allows the preservation of the patternis equally applicable, for example a methane and hydrogen reactive ionetch (RIE) may be used for InP based materials, whereas other plasmasmay be more applicable for GaAs materials. As an alternative to using asingle etching process to form the topographical feature (i.e. the ridgeand slot) and cleave marking notch, the layer of resist material formingthe cleave marking region and the topographical feature may be used as aguide for subsequent processes. Thus for example it is conceivable thatthe etching process (111) is replaced by two distinct process, the firstoperating on the cleave marking region with a second process operatingon the ridge region. The important factor is that the topographicalfeature and cleave features are outlined in the same process and arethus registered with respect to one and other.

The next stage 112 in the process is the application of a layer ofdielectric material. The next step 113 involves opening the top of theridge and the cleave region 120 using a suitable technique. This stepmay also open the region adjacent to the notch. Accordingly, a layer ofdielectric material is absent at and adjacent to the cleave markingnotch.

A subsequent non selective dry etch step is performed to etch 114through the InGaAs(P) active layers, but because of the dielectric thisis confined to regions adjacent to the notch, i.e. the dielectric actsas a resist layer. The effect of the second etching process is to form atiered notch, the lower tier having been formed by the action of thefirst and second etching processes with the upper tier formed by thesecond etching process. The lower tier extends into the substrate,whereas the upper tier remains in the epitaxial layers. The final step115 in the process is a crystallographic etch for InP, which will definea cleave feature, by finding a crystal axis. In certain circumstances,the use of a crystallographic etch on its own may be sufficient and thesecond etching process could be omitted. As the initial cleave markingresist region was formed using the same mask as was used for forming theslot in the ridge, it will be clear that the resulting cleave featurewill be self aligned with the slot. Accordingly, as the cleave featureis formed within the marking notch, it is clear that it will be alignedwith the slots in the ridges formed on the semiconductor body. Thus thissecond aspect of the invention provides for accurate facet or lengthpositioning of topographic features such as a slot with respect to theoverall length of a semiconductor or laser device, along the ridge of aridge laser device.

The invention also provides a method to independently control thespatial and spectral behaviour of the laser, with additional steps itmay also be used to improve the length definition. One problem withcurrent methodologies is that the etch depth for the slot must be chosento be the same as the structure defining the spatial mode. This meansthat there is not true independent control between the spatial andspectral performance. A large ridge is required to maintain spatial modeperformance, while large slot depths can lead to gain loss. Thereforeindependent control of the ridge depth is required to optimise laserperformance. Accordingly, s laser performance is currently compromised.

A manufacturing process to independently control these aspects is shownin FIG. 13 with FIG. 14 reflecting the outcome of each step of theprocess. By incorporating some steps for the cleaving feature, themethod also simplifies the process for registration (fixing thedistance) between the defect (slot) and the cleave feature (and hencethe facet). Although, it will be appreciated that these steps are notessential to achieve independent control of the spatial and spectralbehaviour of the waveguide (laser). The first step (200) comprises thedefinition of a ridge 300 in a resist material layer placed on the topsurface 301 of the epitaxial layers of the wafer, and the definition ofa defect region 302 in the ridge, the defect region being an areadefining ridge section substantially wider than the normal width of theridge. The resulting resist pattern is shown in FIG. 12 a.

The next step (210) is establishing a ridge structure by etching. Theridge may be formed by wet or dry etching or a combination of both. Byusing dry etching for most of the etch sequence, it is easier topreserve the pattern. However, a final wet etch in conjunction with anepitaxially grown etch stop layer allows very accurate depth definition.The use of the etch stop layers in the epitaxial structure effects aprecise epitaxially defined definition of the ridge 305 and defectregion 306.

The next step (220) is to form the outline of a defect (slot) feature inthe layer of resist material covering the structure using a suitablemask and lithographic process. Simultaneously, a cleave marking featuremay be formed in the layer of resist material.

As the slot depth is obtained in a separate step to that step whichdefines the height of the ridge, independent control of the spectral andspatial features of the laser device may readily be achieved, with theridge height providing control of the spatial mode of the laser and theslot depth providing control of the wavelength performance of the laser.

The subsequent step 230 is an etching process that forms a slot 310 inthe defect region 306 to the desired depth. At the same time an openingfor the cleave feature 320 is formed. As the opening for the cleavefeature and slot are formed using the same mask, accurate reproductionof facet/slot distances may be achieved. Following a masking of thestructure in a dielectric material the formation of the cleave featureis continued using a further etching process for example using dryetching through the active layers to the substrate so as to expose theInP substrate, which is then HCl etched.

The remaining steps in the process are standard and have been describedpreviously.

The process flow of FIG. 13 is advantageous in that it eases therequirements on lithography and effects superior lasers. It will beappreciated that Steps 200-230 are applicable to all types of lasers andthat the specific compounds referred to in the definition of the cleavemark are specific to InP-type lasers. It will also be appreciated thatthe process steps described for ridges and slots are equally applicablefor any general topographic feature or means of effecting index or gainchanges. It will be furthermore appreciated that the process describedfor ridge waveguides may also be applied with minor modification toother, more general, semiconductor structures.

An alternative technique to the method steps 200 to 230 of FIG. 13, isprovided in FIG. 14. Although, the process of FIG. 13 providessignificant advantages in the manufacture of laser devices. It may inpractise be difficult to accurately control the depth of the slot in thesecond etching process. This is because when attempting to observe thedepth of the slot, it has been found that the slot is obscured by thesurrounding material in the defect region.

The method of FIG. 15, commences with the step (400), which comprisesthe definition of the ridge in the resist material layer placed on thetop surface of the epitaxial layers of the wafer as before. The step(400) also provides the definition of a defect region in the ridge, thedefect region being an area defining ridge section substantially widerthan the normal width of the ridge. At the same time a gap is left inthe resist material in the defect region. This gap will form an aperturein the defect region when etched.

The next step (410) is the establishing of the ridge structure byetching. However, unlike the previous etching step associated with FIG.13, the height of the established ridge is less than its final value.Moreover, the height of the ridge established equates to the final depthof the aperture in the defect region, which was also formed during thisetching step. The use of the etch stop layers in the epitaxial structuremay be used to effect a precise epitaxially defined definition for thedepth of the slot (initial height of the ridge).

The next step (420) is to form the definition of the ridge in a furtherlayer of resist material layer placed on the top surface of theepitaxial layers of the wafer. The aperture in the defect region ishowever omitted from the definition.

A further etching step (430) is then performed, this etching stepincreases the height of the ridge, but leaves the depth of the aperturein the defect region unaltered. The use of the etch stop layers in theepitaxial structure may be used to effect a definition for the finalheight of the ridge. The remaining steps of the process follow theprevious method of FIG. 13. For ease of explanation, references toforming the cleave feature have been omitted.

As the slot depth is obtained in a separate step to that step whichdefines the height of the ridge, independent control of the spectral andspatial features of the laser device may readily be achieved, with theridge height providing control of the spatial mode of the laser and theslot depth providing control of the wavelength performance of the laser.It will be appreciated that in the method of FIG. 15, the final slotdepth will always be less than the height of the ridge. It will furtherbe appreciated that the methods of FIG. 13 and FIG. 15, provideindependent control of the aperture (slot) depth and the ridge height.

The present invention offers many advantages over prior techniques inthat it provides a method of using projection (optical) lithography tomanufacture devices that were previously achievable only with electronbeam lithography. As such the cost of fabrication is reduced with theresult that devices of the present invention may be applied to a widevariety of applications. The versatility of the technique allows themanufacture of integrated arrays with different wavelengths, for exampleby defining different slot configurations on adjacent ridges.

The present invention provides for a number of applications notpreviously economical using ridge waveguide (laser) devices, including:Optical communications, optical gas sensing, optical metrology anddistance measurement.

For example, in optical communications which refers both to individuallasers in systems where the single frequency aspect is desired, and todense wavelength division multiplexing (DWDM) applications wherechannels are defined by wavelength, the present invention allows thedefinition of different wavelength side by side in an integrated arrayformat. Such an integrated array format could be used in an opticallaser system, in which each ridge laser of the array is opticallycoupled to an associated light guide, e.g. a fibre optic, which in turnfeeds the light produced by the laser devices to an optical multiplexerwhich is coupled to the associated light guides. The multiplexer isadapted to receive the light outputs from each of the ridge lasers andto combine these light outputs into a single combined light output. Thiscombined output may then be used in a variety of applications whereindependent control of multiple wavelength light is required in a singlefibre, e.g. DWDM.

In addition the single frequency aspect can be used as an integratedpump laser to control its spectral properties.

Another important application is in the generation of a high frequency(GHz-THz) optical pulse train for high speed opto-electronic systems.Harmonic mode locking can be generated through the addition of saturableabsorbing regions within a laser cavity containing sub-cavities definedby the slots.

The words “comprises/comprising” and the words “having/including” whenused herein with reference to the present invention are used to specifythe presence of stated features, integers, steps or components but doesnot preclude the presence or addition of one or more other features,integers, steps, components or groups thereof.

1. A semiconductor device formed on a substrate having a plurality oflayers, the device comprising at least one waveguide established by theselective removal of sections of at least one of the layers, thewaveguide comprising at least one elongate section having at least onedefect defining region, the at least one defect defining region of thewaveguide defining an associated defect for the waveguide, wherein thewidth of the waveguide is greater in the at least one defect definingregion of the waveguide than in at least one adjoining section of thewaveguide.
 2. A semiconductor device according to claim 1, wherein thedefect defining region comprises at least one side portion extendingperpendicular to the longitudinal axis of the elongate body portion. 3.A semiconductor device according to claim 2, wherein the defect definingregion has two side portions extending in opposite directions from theelongate body portion of the waveguide.
 4. A semiconductor deviceaccording to claim 1, comprising at least one pair of opposing T-shapedwaveguide sections disposed along the longitudinal axis of the device.5. A semiconductor device according to claim 1, wherein the associateddefect comprises an aperture formed in the waveguide.
 6. A deviceaccording to claim 5, wherein the aperture extends transversely from oneside of the waveguide to the opposing side of the waveguide.
 7. A deviceaccording to claim 5, wherein the aperture is substantially limited inwidth to the width of the elongate portion.
 8. A device according toclaim 5, wherein the height of the waveguide is substantially the sameas the depth of the aperture defined in the defect defining region.
 9. Adevice according to claim 5, the depth of the aperture defined in thedefect defining region is substantially different to the height of thewaveguide.
 10. A device according to claim 9, wherein the depth of theaperture is less than the height of the waveguide.
 11. A deviceaccording to claim 1, wherein the defect is formed by selective IonImplantation in the region of the defect.
 12. A device according toclaim 1, wherein the defect is formed using an Impurity Induced LayerDisorder (IILD) technique selectively in the defect region.
 13. A deviceaccording to claim 1, wherein a plurality of waveguides are formed onthe same substrate.
 14. A device according to claim 13, wherein each ofthe plurality of waveguides is designed by careful selection of defectpositioning and/or size, to provide different operating characteristics.15. An optical system, comprising the device of claim 14, wherein eachwaveguide is optically coupled to an associated light guide, furthercomprising an optical multiplexer coupled to the associated light guidesand adapted to receive the light outputs from each of the waveguides andto combine the light outputs to provide a combined light output.
 16. Adevice according to claim 1, wherein the device is a laser.
 17. A deviceaccording to claim 1, wherein the waveguide is a ridge. 18-36.(canceled)